1. Technical Field
The present invention relates generally to communication systems and in particular, to a method for managing a shared transmission medium, based on a Time Division Multiple Access/Time Division Duplex (TDMA/TDD) scheme between a plurality of stations in a wireless local area network having an Access Point (AP).
2. Related Art
In the OSI (“Open System Interconnect”) model of ISO (“International Standardization Organization”), the information is exchanged between layers on a per packet basis. Each packet received in a given layer is encoded with a set of encoding parameters specific to this layer. These parameters should be given through network administration means. The OSI model specifies a Data Link Layer which is in charge of sharing the physical resource between a plurality of stations. It is composed by two sub-layers namely the Logical Link (LL) sub-layer and the Medium Access Control (MAC) sub-layer. In a TDMA scheme, the MAC sub-layer may control the time period allocated to a given station for emission as well as the PHYsical layer (PHY) encoding parameters in order to provide a better service to the applications running on this station. The MAC sub-layer handles a set of MAC signalling messages to allow a plurality of stations to participate for sharing the physical resource, this set of messages being dependant on the MAC protocol specifically used.
The MAC protocols are able to manage either a distributed allocation scheme such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), which is a contention access scheme, or a centralised allocation scheme such as TDMA for instance.
A CSMA/CA capable station first listens to the medium and transmits packets only if the medium is free. If the medium is busy, the station retries access after a random back-off time period. To secure the packet transmission, the station waits for an acknowledgement from the receiver for each emitted packet. It results there from this that the CSMA/CA scheme introduces guard time and random back-off intervals during which the medium remains unused causing a waste of transmission resource even when there is no collision. In addition, the resulting contention exponentially increases with the number of stations in the network. Besides, CSMA/CA does not provide any guarantee on the transmission delay or on Quality of Service constraints, since no bandwidth reservation scheme is possible with this access scheme. Moreover, the stations must permanently listen to the medium because they may receive a message at any time. In fact, they decode all received messages in order to filter messages intended for another station. It results from this feature that some functions are difficult to implement, e.g. the power saving function, which can be desirable in the context of battery powered wireless stations.
On the contrary, in a MAC layer based on a centralised allocation of network transmission resource such as a Time Division Multiple Access/Time Division Duplex (TDMA/TDD) access scheme, the resource is divided in time periods that are respectively allocated to a particular station, the allocation being performed by a dedicated network entity. This type of access scheme allows to enforce some QoS constraints such as a minimum throughput or a maximum delay, and to enable advanced power saving functions. However, in such a centralised access scheme, it is difficult to achieve a fast reactivity between the time when a station requests transmission resource and the time when the transmission resource is actually allocated to the station. This is due to the complexity of the MAC protocol needed to achieve a centralised allocation scheme, which induces an important signalling overhead. This type of access scheme is better adapted to cellular network architectures, wherein the centralised allocation entity comprises also a gateway function to an external network. As a consequence, it is not really suitable for managing direct transmission between stations.